1. Field of the Invention
The present invention generally relates to a pulse position based-chaotic modulation (PPB-CM) communication system and method. More particularly, the present invention relates to a PPB-CM communication system and method for determining the data bits of a communication signal more accurately, with a simple implementation.
2. Description of the Related Art
The IEEE 802.15.4a task group is the location-aware low-power sensor network standardization group. Location-aware low-power sensor networking is a next generation communication technique in which the location awareness capability and low power requirements are added to the combination of IEEE 802.15.4 ZigBee and IEEE 802.15.3 ultra wide band (UWB) communication techniques.
A chaotic signal modulation method is suggested for the realization of the low power requirement. Chaotic signal modulation can be designed in a simple radio frequency (RF) structure by hardware, and does not require circuitry such as a voltage controlled oscillator (VCO), phase locked loop (PLL), mixer and the like that have been requisite for existing RF products. Thus, chaotic signal modulation can reduce the power consumption to 5 mW, which is one third of that previously considered necessary.
Representative modulation schemes of the chaotic signal modulation are the differential chaos shift keying (DCSK) scheme and the chaotic on-off keying (COOK) scheme.
FIG. 1 is a block diagram of the DCSK communication system. As shown in FIG. 1, the DCSK communication system includes a transmitter 10 and a receiver 20.
The transmitter 10 includes a chaotic signal generator 11, a multiplier 13, a delay 15, and a switch 17. The transmitter 10 loads data on a chaotic signal and transmits the chaotic signal to the receiver 20.
The chaotic signal generator 11 generates a chaotic signal having certain characteristics to carry data.
The multiplier 13 receives a data bit 0 or 1, which is to generate the data, multiplies the chaotic signal generated at the chaotic signal generator 11 by the data bit, and feeds the product to the delay 15. When the data bit is 0, the chaotic signal is reversed, and when the data bit is 1, the chaotic signal is retained.
The delay 15 generates the data signal contained in the second half of the symbol duration by delaying the signal generated at the multiplier 13 by half of the symbol period.
The switch 17 includes a first contact to the chaotic signal generator 11 and a second contact to the delay 15. The switch 17 switches to output one of the signal from the chaotic signal generator 11 and the signal from the delayer 15. Under control of a controller (not shown), the switch 17 switches between the first contact and the second contact each ½ of the symbol period Ts. For instance, when the switch 17 connects to the first contact for ½ Ts, the reference signal is output from the chaotic signal generator 11. When the switch 17 connects to the second contact for ½ Ts, the data signal from the delayer 15 passes through the switch 17 and is output.
The receiver 20 includes a delay 25, a multiplier 23, a waveform generator 27, and a data determiner (not separately shown).
The delay 25 delays the communication signal received via an antenna, by as much as the delay 15 of the transmitter 10 has delayed, that is, by ½ Ts. This is to allow for determining digital data by comparing the reference signal and the data signal.
The multiplier 21 adds the signal delayed at the delay 25 and the communication signal received via the antenna, and provides the added signal to the waveform generator 27. At this time, when the reference signal and the data signal are the same, that is, the data bit is 1, the communication signal with double energy is output. When the data bit is 0, the communication signal with double negative energy is output since the reference signal and the data signal are contrary to each other.
The waveform generator 27 takes the chaotic signal apart and generates a waveform by adding the communication signal output from the multiplier 23 over a certain interval, for example, the symbol period.
The data determiner receives the waveform from the waveform generator 27 and extracts digital data from it. The data determiner determines the digital data according to whether the waveform is over or under a predefined threshold. When the waveform is over the threshold 0, the data determiner determines the digital data as a 1. When the waveform is under the threshold, digital data of 0 is determined.
However, the DCSK communication system needs a lengthy delay line (20 m) to delay the reference signal by ½ Ts when generating the data signal. Hence, the DCSK communication system is not suitable for the IEEE 802.15.4a environment which aims to provide a sensor network.
To overcome this shortcoming, the COOK modulation technique which does not include a delay line is suggested.
FIG. 2 is a block diagram of a COOK communication system.
A transmitter 60 according to the COOK modulation scheme includes a chaotic signal generator 61 for generating a chaotic signal, and a multiplier 63 for multiplying the chaotic signal by the data bit 0 or 1 to generate data. A communication signal output from the transmitter 60 does not carry the chaotic signal in the symbol period having the data bit 0, but carries the chaotic signal in the symbol period having the data bit 1.
A receiver 70 includes a squarer 71, an adder 73, and a data determiner 75.
The squarer 71 squares and outputs the received communication signal. The adder 73 calculates an energy value of the communication signals by summing up all the communication signals within the symbol period.
The data determiner 75 determines whether the data bit contained in the symbol period is 0 or 1 using a threshold. Particularly, the data determiner 75 determines the data bit 1 for the symbol period having the energy value greater than the threshold, and the data bit 0 for the symbol period having the energy value smaller than the threshold. At this time, the data determiner 75 needs to set the threshold. Yet, it is hard for the receiver 70 to predefine the threshold because it does not know the magnitude of the communication signal received from the transmitter 60. In more detail, the receiver 70 has difficulty in acquiring an optimal threshold that is defined to be half of the summation of the signal energies when 1 is received from the transmitter 60 and the signal energies when 0 is received from the transmitter 60. As a result, the accuracy is degraded in relation with the data bit determination.
To resolve the shortcoming of the COOK modulation, a method is suggested to insert a guard interval having no signal between adjacent symbol periods and to set the threshold to a multiple of the real number of the noise energy acquired in the guard interval.
However, it is hard to measure merely the pure noise because the channel power affects the guard interval as the communication signal passes through the channel in the channel environment. Thus, a simulation of the method using the noise energy of the guard interval shows a performance degradation when the noise energy is greater than a specific number, and the error flow when the noise energy is smaller than the specific number.
Therefore, there is a demand for a new communication system capable of accurately determining the data bit of the communication signal, without using a delay line.